It depends on how the clip was made...
If the clip was a 'native' clip, (i.e. the entire clip was created by iMovie itself when importing,) then you should be able to copy/paste and/or find it in the file structure itself.
If the clip was 'created', (i.e. it was part of a longer clip, and you split it within iMovie,) then you're out of luck. I have yet to figure out how to cut the size of a large clip down to get it to transfer to another project.
(In my case, I had imported a DV tape on my old Windows computer. The resulting DV file was 8GB. The one file had about four different 'events' on it, so I wanted to split it up into four parts for four separate iMovies. Well, even though I cut the file down, deleted the unneeded parts in each project, then emptied the trash, each project is a full 8GB. Even the project that now only contains 2 minutes of video.)